In a packed room at the end of a maze of painted brick corridors of the Capitol, 23 conservative Democrats -- most from the South -- openly broke ranks with their party on Tuesday and announced the formation of a policymaking group called, simply, the Coalition.

There are enough moderate and conservative Democrats in the House to triple the size of this group if they all joined. As a united, organized force, they could prove devastating to a Democratic Party that still seems to be searching for its soul.

As a bloc, centrist Democrats are a double-edged sword, perhaps ensuring the success of the Republican agenda, in some form, and undermining the veto just when President Clinton seems to have discovered it.

They have already helped the GOP pass a moderate version of the balanced budget amendment. And after many fellow conservative Democrats fell in November, few are prepared to go to the mat for the president.

''In 1992, [Clinton], as the principal agenda-setter, pulled southern Democrats to the left,'' observes John Cogan, a senior fellow at the Hoover Institution at Stanford University and co-author of a recent study of the November election results.

''But the split [within the Democratic Party] has widened significantly in the last two years, and conservative Democrats may be closer in policy preference to the Republicans,'' he says.

Members of the Coalition squirm when asked if their movement signals a deep rift in the Democratic Party. They claim a bipartisan spirit and have opened their club to all (no Republicans have joined yet).

But they are clearly dissatisfied with the direction of their own party, and the new organization gives them a vehicle for acting as a cohesive, moderate influence on social and economic policy at a time when many Republicans and Democrats have shifted to the extremes.

''We needed to be more forceful in pushing our issues,'' say Rep. Gary Condit (D) of California, a Coalition co-chairman. ''Our intention is to force the Democratic Caucus to deal with us and our issues.''

Bloc voting by centrists on both sides of the aisle, some political observers speculate, may even save the Republicans from themselves by sanding the radical edges off their agenda.

''The centrist bloc will end up benefitting the Republicans,'' predicts Gwen Daye Richardson, co-founder of the conservative, Houston-based Minority Mainstream. ''But if the Republicans choose a pro-life ticket in 1996, the centrists may swing to Clinton.''

The Coalition marks the next chapter in the uneasy alliance between conservative Southern Democrats and the Republican Party that has ebbed and flowed during the past 60 years. The conservative bloc collapsed in the 1960s and 1970s, reappeared in the early 1980s, when ''Boll Weevil'' Democrats supported President Reagan's budget policy, and faded again as the decade progressed.

But what is emerging now may be very different from the previous conservative coalitions. First, the ascendancy of the Republican Party has given conservative Democrats new-found political muscle.

The Coalition, Congressman Condit says, stems in part from the frustration conservative Democrats felt under powerful Democratic committee chairmen for decades. As 14-year Rep. Ralph Hall (D) of Texas says, ''for the first time I'm voting with the winners.''

Second, since the November elections, siding with the Republicans has as much to do with political survival as it does with ideology. As Mr. Cogan's study showed, conservative Democrats lost heavily in the South and West if their records showed they sided with Clinton, especially on economic matters.

Nationwide, the probability of reelection for conservative Democrats in the November election was 47.7 percent, compared with 76.2 percent for liberal Democrats. Consequently, Democrats from conservative districts are much more aware of the price to be paid for taking the party's liberal line.

The Coalition also differs from previous conservative blocs in its organization. Echoing congressional organization, it has three co-chairmen, three regional vice-chairmen, a whip to build bloc voting, and nine issue-based task forces to coordinate united policy stands on everything from welfare reform to taxes.

In the short-run, centrist Democrats are more likely to influence economic rather than social issues, although Condit says the Coalition is working on its own welfare-reform proposal.